The present invention generally relates to axial fans and, more particularly, provides an effective method of reducing unstable stall flow characteristics for the full range of axial fans, including axial fans with hub diameters that are 50 to 90% of the impeller blade tip diameter.
Stall originates when the air cannot accommodate itself to the suction surface of a blade and the air separates from the blade. There are several types of stall that can occur in an axial fan. One type is blade stall that occurs at the hub or blade tip. Typically, stall first occurs at the tip. A stall cell is initiated by reducing the airflow through an impeller below its original design conditions, thereby increasing the air incidence angle into the blade. A stall cell typically occurs when the blade incidence angle exceeds about 8 to 15 degrees. For purposes of illustration, and in the context of three blades A, B, and C, blade B may stall. Substantial cell blockage occurs between blades B and C. Due to the blockage, inlet flow is diverted away from the inlet to B and towards C. The result is an increased angle of attack on C and a reduced angle of attack on A. Since C was on the verge of stalling, it will now stall, whereas A will have less of a tendency to stall.
The above breakdown of the flow into stalled and unstalled sectors or cells is called rotating stall. The stalled cells have low axial velocity, or even negative velocity, whereas the unstalled cells operate at a level of axial velocity consistent with unstalled flow. The stall cell will then propagate along the blade row in the direction of rotation.
There may be one or more rotating stall cells which propagate around the circumference of the impeller with a constant rotational speed, usually between 20 to 70% of the rotor speed. In the cells, the blades are severely stalled. Typically, there is negligible net through-flow with areas of local reverse flow. The cell can vary from covering only part of a blade to over 180 degrees of the annulus. The inception of rotating stall occurs at the peak (zero slope point) of the pressure curve.
In a case type of stall, as the fan approaches stall, the centrifuged low momentum air and reverse airflow build up at the impeller tip and stall the tip.
Efforts to minimize or eliminate stall have been many in the past. For example, U.S. Pat. No. 5,551,841 discloses an axial fan for a hair dryer that seeks to reduce the leakage swirl at the outer peripheral tip edges of the vanes. The fan includes an outer casing and a coaxially telescoping inside wall member, which together form an annular flow path between them. The annular flow path communicates with a second inlet port that is separate from a first inlet port that receives a main air into the fan. The annular flow path is upstream of the vanes of the fan and separate from the main air path. The peripheral air flows through the annular flow path and is directed towards the outer peripheries of the vanes to prevent leakage swirl at the tips of the vanes. Disadvantages, however, include the fact that reducing the backflow leakage at the blade tip does not have any influence on rotating stall cells. This device evidently only works in applications where the fan is blowing into a duct or plenum. The natural leakage path through the annular flow path entrance prohibits this device from being used on a closed loop system or a system where the fan is exhausting from a duct or plenum.
In a past attempt at reducing stall in the context of an axial flow gas turbine high compressor, U.S. Pat. No. 5,607,284 provides an abradable tip shroud assembly intended to address the problem of reduced axial momentum at the blade tips, but with reduced manufacturing costs. The assembly includes an annular shroud extending circumferentially about the longitudinal axis. The annular shroud comprises a plurality of shroud segments having first and second arcuate members with a baffle fixed between them. A layer of an abradable material is positioned intermediate the arcuate members and the blade tip. The arcuate members form a passage that extends from a position radial to the tip of a blade, past the baffle, and then to a position forward of the blade. While providing advantages, some of the disadvantages include an expensive method of stall treatment with only minimal stall improvement. This patent is focused on controlling blade tip gap as the main means of controlling stall. The location of the annular stall cavity and its size, in respect to the axial length of the blade tip, are the main reasons this device only provides minimal rotating stall improvement. The straight baffle in the return air path only provides structural support. The baffle does not recover any of the swirl energy from the air flowing leaving the blade and going through the treatment area.
In U.S. Pat. No. 5,230,605, a prior art air separator in an axial flow blower was described as having a ring supporting a straightening vane, both of which were forward of a rotor vane. A stall zone occurring at the tip of the rotor vane was sucked into a rotor vane tip opening of the housing. The vane tip opening was located radial to and upstream of the rotor vane. The swirling motion of the sucked air was eliminated as it passed through the straightening vane that was disposed in the vane tip opening. The air was then returned to the main air at a position upstream of the rotor vane. The improvement to the prior art design included the straightening vane at a rearward area of the ring. An inlet guide vane was added at the ring and upstream of the rotor vane, whereby the guide vane could be rotated about an axis perpendicular to the longitudinal axis. Some of the disadvantages of this design include the need for an additional fan inlet guide vane. This type of treatment appears to only provide minimal stall improvement for fans with high hub-to-tip ratios of about 50% or less. Due to stall cavity vane location and shape, only minimal recovery (i.e., about 50%) of swirl energy in the air going through the stall cavity is achieved. The amount of blade exposure and the lack of an impeller tip seal are additional reasons this device is ineffective on high hub-to-tip ratio fans.
The axial fan in U.S. Pat. No. 4,871,294 is somewhat akin to the prior art design mentioned in U.S. Pat. No. 5,230,605. The housing forms an annular chamber upstream of the rotor blades and that allows a stalled air to flow from the rotor blade tips and back into a main air upstream of the blades. Also upstream of the rotor blades is a ring that supports at its upstream portion guide vanes within the annular chamber. Disadvantages in this design include minimal rotating stall improvement for fans with high hub-to-tip ratios. The stall cavity vane location and shape only provide minimal recovery (i.e., about 50%) of swirl energy in the air going through the stall cavity. The amount of blade exposure, the lack of an impeller tip seal, and the lack of a diverter are additional reasons this device is ineffective on high hub to tip ratio fans.
Other related art is found in U.S. Pat. Nos. 4,673,331; 4,630,993; 4,602,410; and 3,189,260; as well as M. Ziabasharhagh et al., Presentation at the Intern'l Gas Turbine and Aeroengine Congress and Exposition, Germany (1992).
As can be seen, there is a need for an improved axial fan. Another need is for an axial fan and method that minimizes air stall characteristics. A further need is for an axial fan and method that recirculates an air stall flow back into a main air flow. Also needed is an axial fan and method that reduces air stall cell zones in a simple yet efficient fashion.